Radio-Frequency Identification (RFID) systems typically include RFID readers, also known as RFID reader/writers or RFID interrogators, and RFID tags. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an item or to its package.
RFID techniques include an RFID reader interrogating one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field.
An RFID tag typically includes a substrate, an antenna formed on the substrate, and an integrated circuit (IC) coupled to the antenna. Some RFID tags employ an energy storage device such as a battery and are known as active or battery-assisted tags. Other RFID tags do not include an energy-storage device such as a battery and are called passive tags. Passive tags rely on energy extracted from the RF wave to power the IC. Of course, even passive tags include temporary energy-storage elements such as capacitors or inductors. Regardless, the efficiency of the RF power transfer from the reader to the passive tag's IC directly affects the operational range of the RFID system.
An RFID tag may include a matching network coupled between the IC and the antenna. The purpose of this matching network is to match the antenna impedance to the IC input impedance to maximize power transfer to the IC. This matching network may employ discrete elements such as capacitors or inductors, or may be formed by the design of the antenna itself. To ensure that as much of the RF power incident on the antenna is transferred to the IC, the source (antenna) impedance should be the complex conjugate of the load (IC) impedance. Unfortunately, antenna impedance may vary with environmental conditions such as humidity, substrate material, dielectric materials near the tag, etc. Similarly, IC impedance may vary with the processing of the IC itself. A matching network that matches antenna and IC impedances for one tag and one frequency under one condition will not match antenna and IC impedances for another IC at another frequency under a different condition. When the impedances vary the matching between the antenna and IC degrades, tag sensitivity degrades, and the operational range of the RFID system is reduced. Consequently, there is a need for ways to tune the IC input impedance to be the complex conjugate of the antenna source impedance under varying conditions.